1. Field of the Disclosure
This disclosure relates generally to the field of multidirectional antenna design for well logging systems, tools and methods.
2. Description of the Related Art
Wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of jointed tubulars or a continuous coiled tubing that has a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to its bottom end. The BHA typically includes a number of sensors, formation evaluation tools, and directional drilling tools. A drill bit attached to the BHA is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbore. Logging tools using antennas have been commonly used for determination of formation properties. For example, an electromagnetic wave propagation logging tool for determining electrical properties of the formations surrounding the borehole is often deployed in the BHA. Such tools are generally referred to in the oil and gas industry as the resistivity tools. These logging tools make measurements of apparent resistivity (or conductivity) of the formation that, properly interpreted, provide information about the petrophysical properties of the formation surrounding the borehole and fluids contained therein. Resistivity logging tools also are commonly used for logging wells after the wells have been drilled. Such tools are typically conveyed into the wells by wireline. The tools that use wireline are generally referred to as the wireline resistivity tools, while the logging tools used during drilling of the wellbore are generally referred to a the logging-while-drilling (LWD) or measurement-while-drilling (MWD) tools. These resistivity logging tools also are referred to as induction logging tools. For the purpose of this disclosure, the term resistivity tool or induction logging tool is meant to include all such and other versions of the resistivity tools.
Another type of sensor commonly used in formation evaluation is the nuclear magnetic resonance (NMR) tool. In such tools, a static magnetic field is used to align nuclear spins in a region of examination. Upon excitation with an RF magnetic field, these nuclear spins produce electromagnetic signals that are detected by an antenna and, upon further analysis, can provide information about formation properties such as porosity, fluid saturation, diffusivity, and fluid distribution in the pore spaces.
A typical resistivity tool includes one or more receiver coils or antennas spaced from each other and one or more transmitter coils or antennas. Alternating current is passed through the transmitter coil, which induces alternating electromagnetic fields in the earth formation surrounding the wellbore. Voltages are induced in the receiver coils as a result of electromagnetic induction phenomena related to the alternating electromagnetic fields in the formation.
LWD resistivity tools, for the most part, make omni-directional measurements. The portion of the formation that affects the signals typically takes the shape of a torus. The antenna configuration used in these tools usually includes a number of axial slots (along a longitudinal axis of the tool and the wellbore) made in the tool body. An antenna wire loop is made by placing a wire (electrical conductor) over the slots, perpendicular to the tool longitudinal axis. The longitudinal tool axis is also referred to as the “tool axis.” A ferrite material is often placed in the slots below the wire to increase the efficiency of transmitting antennas or to increase the sensitivity of receiving antennas.
Such axial antennas are also used in NMR measurements. An important aspect of using antennas for both NMR and resistivity measurements is that of protecting the antennas from the hostile borehole environment while, at the same time, allowing the antennas to transmit and/or receive electromagnetic signals. For example, FIG. 2 shows a cross sectional view of an axial antenna for NMR measurements from U.S. Pat. No. 6,838,876 to Kruspe et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. Shown therein is a cross section of the antenna cover 200. The antenna cover 200 is made of stainless steel 5 millimeters thick 202 with 10-millimeter wide slots 204 separated by a 10-millimeter wide rib 206 between each slot 204. The surface 208 of the stainless steel antenna cover 200 is galvanized with copper to reduce resistance and reduce losses from induced eddy currents. The antenna cover slots 204 may be filled with any material 205, for example, rubber, reinforced plastic, epoxy, or any substance that enables passage of electromagnetic energy through the slots. The slot-filling material may be non-electrically conducting. The ends of the slots may be filled with soft magnetic material 210 such as powdered iron bound in epoxy to increase magnetic permeability at the ends of the slots. While slots are shown in the example, any transmissive section formed in the antenna cover is within the scope of the disclosure. In one embodiment, the slots 204 and ribs 206 circumscribe the circumference of the antenna cover, however, in an alternative embodiment, the slot and ribs can cover less than all of the antenna cover circumference, such as, covering only half or one-fourth of the antenna cover circumference to form a side-looking NMR antenna transmission and reception pattern. Alternatively, some of the slots can be formed and filled with non-RF transmissive material to block RF emissions in order to form a side-looking or beam-forming antenna cover. The slotted antenna cover may also be made from beryllium copper or a copper nickel alloy. These materials are wear resistant and desirable for their ruggedness and resistance to abrasion in the downhole environment.
Resistivity tools also have been developed that are sensitive to the azimuthal direction of a resistivity contrast within the depth of investigation of the tool. In such tools, the antenna wire is not perpendicular to the tool axis. Therefore, the slots for the placement of the ferrite material for such antennas are also not oriented along the tool axis. Such slots are tilted relative to the tool axis and in an extreme case are formed perpendicular to the tool axis, i.e., along the radial direction of the tool body or a housing. These slots are in the form of continuous notches made in the metallic housing. However, continuous slots made into a section of an LWD resistivity tool body, such as in a section of a drill collar, reduce mechanical strength of the tool body, which can result in developing cracks when the tool body is subjected to high bending loads during drilling of a wellbore. U.S. patent application Ser. No. 11/854,882 of Peter et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference, discloses an antenna that includes at least two groups of slots, each group having at least two rows, each row having at least one slot therein; and at least one wire is placed over the slots in each row that is not perpendicular to the tool axis (non-parallel to the radial axis). This is illustrated in FIG. 3.
FIG. 3 shows a perspective view of an antenna carrier 300 that includes a metallic carrier 320 (also referred to herein as a housing, sleeve or metallic member). The antenna configuration shown in FIG. 3 is of an x-transverse antenna, in which circumferential notches made in the metallic member 320 are divided or segmented into slots, each slot containing a suitable high magnetic permeability material, such as a ferrite material. As shown in FIG. 3, the first circumferential notch 310a is made up of an array of “m” spaced apart slots, such as slots 310a1 through 310am, while the last notch 310n is made up of “m” slots 310n1 through 310nm. In the example of FIG. 3, each notch is made in the direction perpendicular to the tool axis (“z-axis), i.e., along the circumferential direction of the carrier 320. Similarly, each of the other notches, such as notch 310b through 310n−1, is made up of similar number of slots formed in the circumferential direction. Thus, in the example of FIG. 3, the antenna includes multiple notches, each containing three slots. In the configuration of FIG. 3, the slots 310a1 through 310n1 (i.e., one slot from each notch along a common direction, which in this particular case is the z-direction) form a first row of slots belonging to the set of slots shown. Similarly slots 310a2 through 310n2 and 310am through 310nm, each respectively forms the second and third rows of slots. The number of notches, slots and rows contained in each antenna is a design choice and thus may vary from one design to another. As disclosed therein, antenna wires can be positioned to have transverse orientation.
Multicomponent resistivity logging tools require antennas with both axial and transverse orientation. See, for example, U.S. Pat. No. 6,466,872 to Kriegshauser et al., having the same assignee as the present disclosure and the contents of which are incorporated herein by reference. U.S. patent application Ser. No. 11/858,717 of Signorelli having the same assignee as the present disclosure and the contents of which are incorporated herein by reference teaches the use of collocated antennas for multicomponent resistivity tools. The disclosure in Signorelli specifies different set of aligned parallel grooves for each component. Such an arrangement may be perfectly satisfactory for wireline applications, but for MWD applications, having a large number of grooves could weaken the drill collar. It should be noted that multicomponent antennas may also be used in NMR tools.
The disclosure herein provides improved tools, system and methods for estimating or determining a formation property downhole.